The present invention relates to a process for the gel or surface postcrosslinking of water-absorbing hydrogels by copolymerization of bis- and poly-2-oxazolidinones.
Hydrophilic highly swellable hydrogels are, in particular, polymers composed of (co)polymerized hydrophilic monomers, graft (co)polymers of one or more hydrophilic monomers on a suitable graft base, crosslinked cellulose ethers or starch ethers, crosslinked carboxymethylcellulose, partially crosslinked polyalkylene oxide, or natural products that are swellable in aqueous liquids, such as guar derivatives, for example. Hydrogels of this kind are used as products for absorbing aqueous solutions in the production of diapers, tampons, sanitary towels and other hygiene articles, and as water retainers in market gardening.
To improve service properties such as diaper rewet and AUL, for example, hydrophilic highly swellable hydrogels are generally subjected to surface or gel postcrosslinking. This postcrosslinking is known to the person skilled in the art and is preferably carried out in the aqueous gel phase or as surface postcrosslinking of the milled and sieved polymer particles.
Crosslinkers suitable for this purpose are compounds comprising at least two groups which are able to form covalent bonds with the carboxyl groups of the hydrophilic polymer. Examples of suitable crosslinkers are diglycidyl or polyglycidyl compounds, such as diglycidyl phosphonate, alkoxysilyl compounds, polyaziridines, polyamines or polyamidoamines, and these compounds can also be used in mixtures with one another (see for example EP-A-0 083 022, EP-A-0 543 303 and EP-A-0 530 438). Polyamidoamines which are suitable as crosslinkers are described in particular in EP-A-0 349 935.
A major disadvantage of these crosslinkers is their high reactivity. Although this is desirable in terms of chemical conversion it harbors a relatively high toxicological potential. The processing of such crosslinkers in the production plant requires special protective measures in order to meet the requirements of the relevant safety provisions and of occupational hygiene. In addition, the use of polymers modified in this way within hygiene articles appears questionable.
Polyfunctional alcohols are also known for use as crosslinkers. For example, EP-A-0 372 981, U.S. Pat. Nos. 4,666,983 and 5,385,983 teach the use of hydrophilic polyalcohols or the use of polyhydroxy surfactants. According to these documents the reaction is carried out at temperatures of 120-250xc2x0 C. The process has the disadvantage that the esterification reaction which leads to crosslinking is relatively slow even at such temperatures.
The object was therefore, using compounds which are relatively slow to react yet which are reactive with carboxyl groups, to achieve gel or surface postcrosslinking which is as good if not better than that of the prior art, with as short as possible a reaction time and as low as possible a reaction temperature. Ideally, the prevailing reaction conditions should be the same as those obtained when highly reactive epoxides are used.
It has surprisingly now been found that this object can be achieved to outstanding effect with bis- and poly-2-oxazolidinones. In particular, the reactivity of these crosslinkers can be increased by adding organic or inorganic acidic catalysts. Suitable catalysts are the known inorganic mineral acids, their acidic salts with alkali metals or with ammonium, and their anhydrides. Suitable organic catalysts are the known carboxylic, sulfonic and amino acids.
The invention provides a process for the surface postcrosslinking of water-absorbing polymers in which the polymers are treated with a surface postcrosslinking solution and during or after the treatment are postcrosslinked by means of an increase in temperature and are dried, wherein the crosslinker comprises a bis-2-oxazolidinone or a poly-2-oxazolidinone comprising structural units of the formula 
in which R1 is branched or unbranched C1-C18-alkylene, branched or unbranched C2-C18-alkenylene, C5-C8-cycloalkylene, phenylene, naphthylene, anthracenylene, hydrocarbon-substituted phenylene, naphthylene or anthracenylene or another substituted or unsubstituted C6-C18-arylene radical, R2 is branched or unbranched C1-C18-alkylene and n is an integer from 1 to 50 or a mixture of bis-2-oxazolidinones and poly-2-oxazolidinones dissolved in an inert solvent.
Where R1 is an alkylene or alkenylene radical it is preferably one having a chain length of from 3 to 12, in particular from 5 to 10 carbon atoms. R2 is preferably an alkylene radical having a chain length of from 3 to 12, in particular from 5 to 10, carbon atoms.
Terminal structural units of the formula 1 are endgroup-capped. The endgroup used can be any radical which can be introduced into the bis-2-oxazolidinones or poly-2-oxazolidinones and is chemically stable on these compounds. Examples of suitable radicals with which the structural units of the formula 1 can be endgroup-capped are hydrogen, branched or unbranched C1-C18-alkyl, branched or unbranched C2-C18-alkenyl, phenyl, naphthyl, anthracenyl, hydrocarbon-substituted phenyl, naphthyl or anthracenyl or another substituted or unsubstituted C6-C18-aryl radical.
The poly-2-oxazolidinones preferably comprise n bis-2-oxazolidinone units.
Preferably n is a number between 1 and 10, with particular preference between 3 and 6. The postcrosslinking temperature is preferably between 50 and 250xc2x0 C., in particular 50-200xc2x0 C. and especially 100-180xc2x0 C.
To accelerate the surface postcrosslinking reaction, an acidic catalyst may be added to the reaction mixture. Catalysts which can be used in the process of the invention are all inorganic acids, their corresponding anhydrides, and/or organic acids and their corresponding anhydrides. Examples are boric, sulfuric, hydroiodic, phosphoric, tartaric, acetic and toluenesulfonic acid. Also suitable in particular are their polymeric forms, anhydrides, and the acid salts as occur, for example, in the case of the polybasic acids. Examples thereof are boron oxide, sulfur trioxide, diphosphorus pentoxide, and ammonium dihydrogen phosphate.
The process of the invention is preferably conducted by spraying a solution of the surface postcrosslinker onto the dry base polymer powder. Following spray application, the polymer powder is dried thermally, it being possible for the crosslinking reaction to take place either before or during drying. Preference is given to the spray application of a solution of the crosslinker in reaction mixers and spray mixers or mixing and drying systems, such as, for example, Lxc3x6dige mixers, (copyright)BEPEX mixers, (copyright)NAUTA mixers, (copyright)SHUGGI mixers or (copyright)PROCESSALL apparatus. It is, moreover, also possible to employ fluidized-bed dryers. Drying can take place in the mixer itself, by heating the outer casing or by blowing in hot air. Likewise suitable is a downstream dryer, such as a shelf dryer, a rotary dryer or a heatable screw. Alternatively, azeotropic distillation, for example, can be utilized as a drying technique. The residence time at the preferred temperature in the reaction mixer or dryer is from 5 to 90 minutes, preferably less than 30 minutes and, with very particular preference, less than 10 minutes.
As an inert solvent, preference is given to water and to mixtures of water with simple or polyfunctional alcohols. It is, however, possible to employ all organic solvents of unlimited miscibility with water, examples being certain esters and ketones, which are not themselves reactive under the process conditions. Where an alcohol/water mixture is employed the alcohol content of this solution is 10-90% by weight, preferably 30-70% by weight and, in particular, 40-60% by weight. Any alcohol of unlimited miscibility with water can be employed, as can mixtures of two or more alcohols (e.g. methanol+glycerol+water). Particular preference is given to the use of the following alcohols in aqueous solution: methanol, ethanol, isopropanol, ethylene glycol and, with particular preference, 1,2-propanediol and also 1,3-propanediol. The surface postcrosslinking solution is employed in a proportion of 1-20% by weight based on the mass of polymer. Particular preference is given to a solution volume of 2.5-15% by weight based on polymer. The crosslinker itself is used in an amount of from 0.01-1.0% by weight based on the polymer employed.
The water-absorbing polymer is preferably a polymeric acrylic acid or a polyacrylate. This water-absorbing polymer can be prepared by a process known from the literature. Preference is given to polymers which include crosslinking comonomers (0.001-10 mol %) but very particular preference is given to polymers which have been obtained by means of free-radical addition polymerization using a polyfunctional ethylenically unsaturated free-radical crosslinker which in addition carries at least one free hydroxyl group (such as pentaerythritol triallyl ether or trimethylolpropane diallyl ether, for example).
The hydrophilic highly swellable hydrogels to be employed in the processes of the invention are, in particular, polymers of (co)polymerized hydrophilic monomers, graft (co)polymers of one or more hydrophilic monomers on a suitable graft base, crosslinked cellulose ethers or starch ethers, or natural products which are swellable in aqueous liquids, such as guar derivatives, for example. These hydrogels are known to the person skilled in the art and are described, for example, in U.S. Pat. No. 4,286,082, DE-C-27 06 135, U.S. Pat. No. 4,340,706, DE-C-37 13 601, DE-C-28 40 010, DE-A-43 44 548, DE-A-40 20 780, DE-A-40 15 085, DE-A-39 17 846, DE-A-38 07 289, DE-A-35 33 337, DE-A-35 03 458, DE-A-42 44 548, DE-A-42 19 607, DE-A-40 21 847, DE-A-38 31 261, DE-A-35 11 086. DE-A-31 18 172, DE-A-30 28 043, DE-A-44 18 881, EP-A-0 801 483, EP-A-0 455 985, EP-A-0 467 073, EP-A-0 312 952, EP-A-0 205 874, EP-A-0 499 774, DE-A-26 12 846, DE-A-40 20 780 EP-A-0 205 674, U.S. Pat. No. 5,145,906, EP-A-0 530 438, EP-A-0 670 073, U.S. Pat. Nos. 4,057,521, 4,062,817, 4,525,527, 4,295,987, 5,011,892, 4,076,663 or 4,931,497. The content of the abovementioned patent documents is expressly incorporated into the present disclosure by reference.
Examples of hydrophilic monomers suitable for preparing these hydrophilic highly swellable hydrogels are polymerizable acids, such as acrylic acid, methacrylic acid, vinylsulfonic acid, vinylphosphonic acid, maleic acid including its anhydride, fumaric acid, itaconic acid, 2-acrylamido-2-methylpropanesulfonic acid, 2-acrylamido-2-methylpropanephosphonic acid, and also their salts in each case, for example sodium, potassium or ammonium salts, their amides, hydroxyalkyl esters, and amino- or ammonium-functional esters and amides. Also suitable, furthermore, are water-soluble N-vinyl amides or else diallyidimethylammonium chloride. Preferred hydrophilic monomers are compounds of the formula 2 
in which
R3 is hydrogen, methyl or ethyl,
R4 is xe2x80x94COOR6, a sulfonyl group, a phosphonyl group, a (C1-C4)-alkanol-esterified phosphonyl group, or a group of the formula 3 
R5 is hydrogen, methyl, ethyl or a carboxyl group,
R6 is hydrogen, amino-(C1-C4)-alkyl or hydroxy-(C1-C4)-alkyl, an alkali metal ion or ammonium ion, and
R7 is a sulfonyl group, a phosphonyl group or a carboxyl group, or alkali metal or ammonium salts of these groups.
Examples of (C1-C4)-alkanols are methanol, ethanol, n-propanol and n-butanol. Particularly preferred hydrophilic monomers are acrylic acid and methacrylic acid and also their alkali metal or ammonium salts, for example sodium, potassium and ammonium acrylate.
Suitable graft bases for hydrophilic hydrogels obtainable by graft copolymerization of olefinically unsaturated acids or their alkali metal or ammonium salts may be natural or synthetic in origin. Examples are starches cellulose and cellulose derivatives, and also other polysaccharides and oligosaccharides, polyalkylene oxides, especially polyethylene oxides and polypropylene oxides, and hydrophilic polyesters.
Suitable polyalkylene oxides have, for example, the formula 
in which
R8 and R9 independently of one another are hydrogen, alkyl, alkenyl or acyl,
X is hydrogen or methyl, and
n is an integer from 1 to 10,000.
R8 and R9 are preferably hydrogen, (C1-C4)-alkyl, (C2-C6)-alkenyl or phenyl. Particularly preferred hydrogels are polyacrylates, polymethacrylates, and the graft copolymers described in U.S. Pat. Nos. 4,931,497, 5,011,892 and 5,041,496.
The hydrophilic highly swellable hydrogels are preferably in crosslinked form; that is, they include compounds having at least two double bonds which have been incorporated by copolymerization into the polymer network. Particularly suitable crosslinkers are methylenebisacrylamide and methylenemethacrylamide, esters of unsaturated mono- or polycarboxylic acids with polyols, such as diacrylate or triacrylate, examples being the diacrylate and dimethacrylate of butanediol and of ethylene glycol, and trimethylolpropane triacrylate, and also allyl compounds such as allyl (meth)acrylate, triallyl cyanurate, diallyl maleate, polyallyl esters, tetraallyloxyethane, triallylamine, tetraallylethylenediamine, allyl esters of phosphoric acid, and vinylphosphonic acid derivatives as are described, for example, in EP-A-0 343 427. In the process of the invention, however, particular preference is given to hydrogels prepared using polyallyl ethers as crosslinkers and by acidic homopolymerization of acrylic acid. Suitable crosslinkers are pentaerythritol tri- and tetraallyl ether, polyethylene glycol diallyl ether, monoethylene glycol diallyl ether, glycerol di- and triallyl ether, polyallyl ethers based on sorbitol, and alkoxylated variants thereof.
The hydrophilic highly swellable hydrogels can be prepared by conventional polymerization processes. Preference is given to addition polymerization in aqueous solution by the process known as gel polymerization. In this process from 15 to 50% by weight strength aqueous solutions of one or more hydrophilic monomers, and, if desired, of a suitable graft base are polymerized in the presence of a free-radical initiator, preferably without mechanical mixing, utilizing the Trommsdorff-Norrish effect (Makromol. Chem. 1 (1947)169).
The polymerization reaction can be conducted in the temperature range between 0xc2x0 C. and 150xc2x0 C., preferably between 10xc2x0 C. and 100xc2x0 C., either at atmospheric pressure or under an increased or reduced pressure. The polymerization may also be performed in an inert gas atmosphere, preferably under nitrogen.
The polymerization can be initiated using high-energy electromagnetic radiation or by the customary chemical polymerization initiators. Examples of the latter are organic peroxides, such as benzoyl peroxide, tert-butyl hydroperoxide, methyl ethyl ketone peroxide and cumene hydroperoxide, azo compounds, such as azodiisobutyronitrile, and inorganic peroxi compounds, such as (NH4)2S2O8, K2S2O8 or H2O2. These can if desired be used in combination with reducing agents such as sodium hydrogen sulfite, iron(II) sulfate, or redox systems. Redox systems include a reducing component, which is generally an aliphatic or aromatic sulfinic acid, such as benzenesulfinic acid or toluenesulfinic acid or derivatives of these acids, such as Mannich adducts of sulfinic acid, aldehydes and amino compounds as are described in DE-C-13 01 566.
The qualities of the polymers can be improved further by continuing to heat the polymer gels for a number of hours within the temperature range from 50 to 130xc2x0 C., preferably from 70 to 100xc2x0 C.
The resultant gels are neutralized to the extent of 0-100 mol % based on monomer employed, preferably 25-100 mol % and, with particular preference, 50-85 mol %, it being possible to use the customary neutralizing agents, preferably alkali metal hydroxides or alkali metal oxides, and with particular preference sodium hydroxide, sodium carbonate or sodium hydrogen carbonate. Neutralization is usually effected by mixing in the neutralizing agent as an aqueous solution or else, preferably, as a solid. For this purpose the gel is mechanically comminuted, by means for example of a mincer, and the neutralizing agent is sprayed on, scattered over or poured on, and then carefully mixed in. To effect homogenization the resultant gel mass may be passed through the mincer a number of times more.
The neutralized gel mass is then dried with a belt or roll dryer until the residual moisture content is less than 10% by weight, preferably below 5% by weight. The dried hydrogel is then ground and sieved, the usual candidates for grinding apparatus being roll mills, pin mills or vibrator mills. The preferred particle size of the sieved hydrogel lies in the range 45-1000 xcexcm, with particular preference 45-850 xcexcm, and, with very particular preference, 200-850 xcexcm.
In accordance with the invention, acrylate-containing polymers are crosslinked using bis- or poly-2-oxazolidinones. The novel crosslinker of the invention can be prepared by reacting isocyanates and diepoxides (bis-2-oxazolidinones) or diisocyanates and diepoxides (poly-2-oxazolidinones).
The general equation for the formation of bis-2-oxazolidinones is as follows: 
The formation of poly-2-oxazolidinones can be described by the general equation for the polyaddition: 
R1 and R2 are as defined above. In the preparation of poly-2-oxazolidinones it is possible to add a certain amount of monoisocyanates to the reaction mixture. These terminate the polyaddition when they are incorporated. The amount of monoisocyanates added must in this case be such as to lead to the desired chain length of the poly-2-oxazolidinones. The radical bearing the isocyanate group is then the end group. In the repeating unit of the polymer two oxazolidinone units have formed as a result of the polyaddition reaction, with n being an integer greater than 1. This polyaddition reaction takes place preferentially but not exclusively in polar aprotic solvents which react neither with the diisocyanate nor with the diepoxide at the prevailing temperature. Aliphatic diisocyanates produce with diepoxides pale products of low melting point which are soluble, for example, in dimethylformamide, whereas the reaction of aromatic diisocyanates generally leads to dark-colored and largely insoluble products of high melting point.
When aromatic diisocyanates are used the products are usually of poor solubility and high melting point, and it is therefore preferred to use aliphatic diisocyanates with aliphatic or aromatic diepoxides.
The invention also provides a product prepared by the process described above.
The invention additionally provides for the use of the products prepared by the process of the invention in hygiene articles, packaging materials and nonwovens.
In order to ascertain the quality of surface postcrosslinking, the dried hydrogel is then tested using the prior art test methods described below:
Methods
1) Centrifuge Retention Capacity (CRC)
This method measures the free swellability of the hydrogel in a teabag. Approximately 0.200 g of dry hydrogel are placed in a sealed teabag (format: 60 mmxc3x9760 mm, Dexter 1234T paper) and soaked for 30 minutes in 0.9% strength by weight sodium chloride solution. The teabag is then spun for 3 minutes in a commercially available spin dryer (Bauknecht WS 130, 1400 rpm, basket diameter 230 mm). The volume of liquid absorbed is determined by weighing the centrifuged teabag. The absorption capacity of the teabag itself is taken into account by determination of a blank value (teabag without hydrogel), which is deducted from the weighing result (teabag with swollen hydrogel).
Retention CRC [g/g]=(weighing result teabagxe2x88x92blank valuexe2x88x92initial weight of hydrogel)÷initial weight of hydrogel
2) Absorbency Under Load (0.3/0.5/0.7 psi)
For the absorbency under load, 0.900 g of dry hydrogel is distributed uniformly on the screen base of a measuring cell. The measuring cell consists of a Plexiglas cylinder (height=50 mm, diameter=60 mm) whose base is formed by sticking on a screen of steel mesh (mesh size 36 microns, or 400 mesh). A cover plate is placed over the uniformly distributed hydrogel and loaded with an appropriate weight. The cell is then placed on a filter paper (SandS 589 black band, diameter=90 mm) lying on a porous glass filter plate, this filter plate itself lying in a Petri dish (height=30 mm, diameter=200 mm) which contains 0.9% strength by weight sodium chloride solution in an amount such that the liquid level at the beginning of the experiment is level with the top edge of the glass frit. The hydrogel is then left to absorb the salt solution for 60 minutes. Subsequently, the complete cell with the swollen gel is removed from the filter plate and the apparatus is back-weighed following removal of the weight.
The absorbency under load (AUL) is calculated as follows:
AUL[g/g]=(Wbxe2x88x92Wa)/Ws
where
Wb is the mass of the apparatus+gel after swelling,
Wa is the mass of the apparatus+initial weight of gel before swelling, and
Ws is the initial weight of dry hydrogel.
The apparatus consists of measuring cylinder and cover plate.